1. Field of the Invention
The present invention relates to a zirconia sintered body having excellent stability of surface smoothness even under hydrothermal conditions. This zirconia sintered body worked precisely is useful widely as precise parts such as optical fiber connector parts (ferrules, split sleeves, etc.), balls for milling machines, pump parts (impellers, casings, mechanical seal sliding rings, bearing balls, sleeves, etc.), semiconductor substrates, biological material parts (artificial bones, artificial joints, etc.), and so forth.
2. Description of the Related Art
The zirconia sintered bodys and the process for production thereof are known. Examples are shown below.
(1) Zirconia ceramics containing Y.sub.2 O.sub.3 and ZrO.sub.2 at a molar ratio ranging from 2/98 to 7/93, constituted of cubic and tetragonal crystal grains having an average particle size of not more than 2 .mu.m, and deteriorating less at a temperature ranging from 200.degree. C. to 300.degree. C. (JP-B-4-63024).
(2) A zirconia sintered body containing 3 to 4 mol % of Y.sub.2 O.sub.3, constituted of 12 to 40% by volume of a cubic phase as measured by Rietveld method and the balance of tetragonal zirconia, containing Y.sub.2 O.sub.3 dissolved in a solid state in tetragonal zirconia at a content of not less than 2.3 mol % as derived from the lattice constant, having an average particle size of not more than 0.5 .mu.m, having a relative density of not less than 95%, and forming a transformed layer at 175.degree. C. for 47 hours in a thickness of not more than 30 .mu.m (JP-A-8-325057).
The zirconia sintered body (1) above is produced by calcining a powdery mixture of ZrO.sub.2 and Y.sub.2 O.sub.3, pulverizing the calcined mixture, molding pulverized matter, and sintering the molded article. The resulting zirconia sintered body produced by this process has many monoclinic crystal grains on the surface of the sintered body owing to nonuniformity of the Y.sub.2 O.sub.3. Such a sintered body, when subjected to a hydrothermal treatment, will deteriorate rapidly from the surface into the interior by tetragonal-monoclinic phase transformation to lose its strength and toughness, becoming unsuitable for applications as a zirconia sintered body. The zirconia sintered body (2) above is produced from a powdery source material having a small specific surface area obtained by co-precipitation by molding and sintering. The resulting zirconia sintered body has a broad crystal grain size distribution, and the surface thereof undergoes readily phase transformation when subjected to a hydrothermal treatment to cause roughness and irregularity of the surface, which is not suitable for applications of the zirconia sintered body.